Silk suture



United States Patent 3,424,164 SILK SUTURE Alfred Bloch, Somerset, andArthur S. Messores, North Brunswick, NJ, assignors to Ethicon, Inc., acorporation of New Jersey N0 Drawing. Filed May 20, 1966, Ser. No.551,536 US. Cl. 128-3355 4 Claims Int. Cl. A611 17/00; D02g 3/40ABSTRACT OF THE DISCLOSURE A non-capillary, braided silk suture isprepared by treating the suture with a solution of fibroin and drying atabout 105 C. whereby the silk filaments are coated with fibroin andbonded together.

The present invention relates to surgical ligatures and sutures and morespecifically to braided or twisted silk sutures that have been treatedto reduce capillarity. Silk is a material widely used in surgery as anon-absorbable suture to sew wounds or as a ligature to tie up bleedingblood vessels. Silk so used is applied most frequently in form of abraided thread, that is, a central core strand surrounded by a tube-likebraided structure. The central core sometimes is absent or filling thetube to a greater or lesser extent. The braided structure also varies intightness, depending on the braiding technique applied. Another type ofsilk thread, namely twisted silk, is used less frequently in surgery. Init a plurality of silk fibers are twisted together. While the presentinvention is directed to the treatment of braided silk, the procedure tobe described below is also applicable to twisted silk.

Braided silk per se is limp. When it is cut, the ends tend to broom;that is, the carriers of which the braid consists spread out,particularly upon pressure or friction such as applied when attemptingto thread the braided strand through the eye of a needle. Threading mayactually become impossible. Furthermore, braided silk exposed toliquids, solutions, or suspensions exercises a capillary or wick action.Through this capillary or wick action, it can imbibe blood or serousfluid when used in the suturing of a wound. If such tissue fluids areinfested with microorganisms, the latter may be harbored within theinterstices of the silk suture. It has been claimed that microorganismscaptured within such silk structure are extremely resistant todestruction, wherefore such silk could present a danger for the surgicalpatient. Braided silk, which has not been treated to eliminate this wickeffect, is called capillary silk and is classified as Type ANon-Absorbable Surgical Silk in the Pharmacopeia of the United States.Type B or non-capillary silk is treated in such a manner as to eliminateor reduce the aforementioned wick action.

Silk sutures may be treated in various Ways to produce a non-capillarysilk. The methods known or previously described make use of somematerial foreign to silk, which is incorporated into the silk strand sothat it adheres to the silk fibers and/or fills interfilamental voids.The method most commonly used in the trade is that of treating the silkwith wax or wax compositions. This method has the advantage of renderingsilk non-capillary and at the same time bonding the'filaments togetherso as to prevent brooming when the ends are cut. It also gives the silka certain texture; that is, a certain stiffness combined withplasticity; which is usually called hand. Many surgeons prefer silk witha hand over limp silk, since the limp silk is more difficult to handle.Another method to render silk non-capillary is to treat it withwater-repellent compounds, such as silicon compounds.

In order to attain these properties, as much as 20 per- Patented Jan.28, 1969 cent to 30 percent by weight of wax composition may beincorporated into the silk. However, it is known that foreign materialsincorporated into the silk to produce noncapillarity, such as waxes, maynot be well tolerated by the host tissues. They are able to evoke markedinflammatory reactions and granuloma. Wax granuloma may form at a slowprogressive rate about the suture material and in the case of a tendonrepair cause difliculties with the function of the tendon after three tosix months.

It has now been discovered that silk sutures and ligatures may betreated to obtain the desired properties of non-capillarity, bonding,and improved hand without adding thereto any foreign material that mighthave an adverse biological reaction on the host tissue.

In accordance with the present invention, silk filaments that have beenbraided or twisted to form a strand are treated with a solution ordispersion of silk to produce a silk strand of reduced capillarity andimproved hand; the interstices of the silk structure being impregnatedwith a silk substance that coats the silk whereby the silk filaments arecoated and bonded together.

Natural silk, as found in the cocoon of the silkworm of commerce (Bombyxman) is composed of two major components; fibroin, the fibrous silk usedas a textile fiber; and sericin, a non-fibrous protein. In silktechnology, the sericin is removed by various ways and means. and onlythe fibroin is utilized for textile purposes. This present inventiondeals with the fibroin. For the purpose of clarity, the term silk shallbe used in the following specification and claims to identify braided ortwisted silk or strands of silk filaments. The term fibroin shall beused to identify a silk composition that has been dissolved or dispersedfor use as a coating or impregnating material. It will be understoodthat the solutions or dispersions employed in carrying out the presentinvention contain fibroin and, therefore, are similar to silk filamentsand strands from the standpoint of their chemical composition.

In the practice of the present invention, silk fibroin obtained fromsilk cocoons, virgin silk, or waste silk is dispersed or dissolved in asolvent. While many solvents are known for silk, a suitable solvent isaqueous lithium thiocyanate as this salt does not materially degrade thefibroin if the pH of the aqueous solution is buffered to a pH of about7. Fifty percent by weight of lithium thiocyanate in water is a veryefficient solvent, in which 20 percent to 25 percent by weight offibroin can be readily dissolved. Such a solution cannot be useddirectly, however, to impregnate silk fibers, since it attacks them veryrapidly. The lithium thiocyanate can be removed from the silk solution,for instance by dialysis. By exhaustive dialysis against water, a fibroin-in-water dispersion is obtained. This fibroin dispersion can beutilized to impregnate and coat the silk. However, a fibroin dispersionis labile in that is gels readily. Fibroin separates from the dispersionafter standing for some time or through agitation. This property isundesirable for manufacturing purposes. An important facet of thepresent invention is that of stabilization of such fibroin solutions,One way of achieving this is to dialize the solution of fibroin inaqueous lithium thiocyanate against an equeous solution of ahydrogen-bond breaker urea or guanidine. The satisfactory removal oflithium thiocyanate from the fibroin solution may be demonstrated by anegative reaction with ferric chloride.

The dialysis of the fibroin solution to remove lithium thiocyanate isnot necessary if the concentration of the lithium thiocyanate is reducedto no more than 20 percent by weight. However, the nature of thedilution is critical. Dilution of the fibroin solution with water orwith a l-molar aqueous solution of urea results in the formation of aflocculent precipitate. The fibroin remains dispersed if a l-molarsolution of calcium chloride is used as a diluent, but the fibroinsolution assumes a milky, white appearance. Dioxane maybe used as adiluent to reduce the concentration of lithium thiocyanate in solutionas may an aqueous solution of 50 parts acetamide dissolved in 50 partsof water. Dilution of the fibroin solution with aqueous acetamide ispreferably carried out at pH 7 from the standpoint of optimum solutionviscosity and minimum degradation of the fibroin.

Aqueous iithium bromide may be substituted for aqueous lithiumthiocyanate as a solvent for fibroin. The lithium bromide solution soobtained is also stable after dilution with aqueous acetamide.

The present invention will appear more clearly from the followingdetailed examples which illustrate preferred embodiments of theinvention idea.

EXAMPLE I The impregnation and coating of silk can be done by variousmethods, such as immersion or preferably by immersion under vacuum inorder to obtain good penetration. After withdrawal from the fibroinsolution, the silk strands are drained or wiped from excess fibroin andair dried with or without tension. The fibroin may be further fixed byvarious means, such as heating or fixation with precipitants, such assalts. These salts, as well as any urea present from the dialysis step,may be removed by bathing the fixed strands in water or a water-alcoholmixture. A fibroin solution containing 20 percent fibroin in 50 percentby weight of lithium thiocyanate is prepared. Aliquot portions of thissolution are dialyzed in cellophane bags for 24 hours against ureasolutions of varying urea concentrations. The dialysis is repeated asecond time against fresh urea solutions of the respectiveconcentrations. The complete removal of lithium thiocyanate isdemonstrated by a negative reaction with ferric chloride of the fibroinsolution within the cellophane :bag. The fibroin solutions, reduced to10 percent concentrations during dialysis, are filtered through sinteredglass filters under negative pressure and aliquots of each solutionstored at room temperature (20 to 25 C.) and at 37 C.

TABLE I Urea concentration against which silk fibroin solution Observedgelation time is dialyzed At room temperature At 37 C.

3 days 1 day. 0.1 molar 3 days. 3 days. 0.5 molar 7 days--. 3 days. 1.0molar 3 weeks. 7 days. 4.0 molar 8 weeks 6 weeks.

Two parts of fibroin fibers obtained from cleaned silk cocoons aredissolved in 10 parts of 50 percent by weight LiSCN. The fibroinsolution is placed into a cellophane bag and dialyzed against 1000 partsof 4-molar urea solution for 24 hours. The solution is then dialyzed asecond time against 100 parts of 4 molar urea for hours. At this time,the fibroin solution gives a negative reaction for the SCN-ion with FeClThe fibroin solution is filtered through a sintered glass filter undernegative pressure and 4 stored at 25 C. A coil of braided silk, U.S.P.size #1, is immersed in the fibroin solution, air dried, and then heatedat 110 C. for 1.5 hours. The silk strand so obtained is non-capillary.

EXAMPLE III Two and one-half parts of silk fibroin obtained from silkcocoons are dissolved in 10- parts of 50 percent LiSC-N. The fibroinsolution is dialyzed against 1000 parts of 1 molar urea for 24 hours;the dialysis is repeated with another 1000 parts of 1 molar urea for 24hours. The fibroin solution is filtered as in Example 11. Five feet ofbraided silk, 'U.S.P. size #1, is immersed in 5 milliliters of thefibroin solution, and the whole placed under vacuum for 2 hours. Theimpregnated silk coil is drained, air dried for about 20 hours, and thenheated for 2 /2 hours at 105 C. The dried sample is immersed for 3 hoursat room temperature in 50 percent by volume aqueous isopropanol toremove urea from the silk. The sample is non-capillary, bonded so as notto broom when out, and has a hand markedly different from the untreatedcontrol.

EXAMPLE IV Twenty parts of waste silk fibers are dissolved in parts of50 percent LiSCN. The fibroin solution is dialyzed against 2000 parts 1molar urea. The dialysis step is repeated. The dialyzed fibroin solutioncontains 10.5 percent fibroin. A coil of braided silk is immersed in analiquot of the fibroin solution under vacuum for 2.5 hours. The silk iswithdrawn and the excess is wiped off with a towel. The silk strands aredried under two different conditions.

(A) A 10 yard strand is dried without tension.

(B) A 10 yard strand is dried under 50 grams tension.

Both silk strands are heated at C. for 3 hours and immersed in 50percent by volume aqueous isopropanol for 3 hours at room temperature.

Drying under tension results in a shrinkage or shortening of 0.4percent; without tension of 1.7 percent. Both are acceptable. Physicalproperties, such as tensile strength, knotting, and knot-holding powerare excellent. The samples are non-capillary.

EXAMPLE V Silk strands are treated by the procedure discussed in ExampleIII, except that the tfibroin solution is dialyzed against a 1 percentsolution of a quaternary ammonium salt, instead of against a 1 molarurea solution. The treatment resulted in non-capillary silk.

EXAMPLE W Silk strands are treated by the procedure discussed in ExampleIII, except that the fibroin solution is dialyzed against 0.1 molarsolution. The treatment resulted in non-capillary silk.

EXAMPLE VII Size 3-0 black, braided, unwaxed, silk sutures areimpregnated by the following method: twenty parts of degummed silkcocoons are solubilized overnight in 100 parts of a 50 percent LiSCNsolution (i.e., 50 parts LiSCN and 50 parts H O) at pH 7.0. The fibroinconcentrate is then dialyzed against 4000 parts of 1 molar urea at roomtemperature for a period of 24 hours. Following dialysis, the fibroinsolution is checked with ferric chloride for complete salt removal andthen filtered through a coarse sintered glass filter under vacuum.

Size 30 black, braided silk is loosely wound on a perforated spool toallow permeation. The spool is placed in a reservoir, and the ureasolution is added to a height suflicient to completely cover the spool.The silk reservoir with spool and fibroin solution is placed in a vacuumchamber overnight with 30 inches of vacuum maintained.

The silk strand is then drawn mechanically from the fibroin bath throughan orifice which controls the amount Diameter mils 9.8 Straight tensilestrength lbs 4.6 Knot tensile strength lbs 2.9 Wet knot tensile strengthlbs 2.3

The structure so obtained is non-capillary, non-brooming, and hasexcellent hand and bonding characteristics.

EXAMP-IJE VIII Size 3-0 black, braided, unwaxed, silk sutures are triplecoated by the process described above in Example VII. The suturefollowing 3-fold impregnation shows the following physical properties:

Average diameter mils 10 Average straight tensile strength lbs 4.8Average knot tensile strength lbs 3.2 Average wet knot tensile strengthlbs 2.3

The final strand is about 2 percent heavier indicating that about 2percent by weight fibroin is picked up by the strands. The sutureexhibits non-capillarity, nonbrooming, and excellent hand and bondingcharacteristics.

EXAMPLE IX Using the impregnating method and fibroin solution describedin Example VII, sizes 2-0, 3-0, 4-0, and 6-0 black, braided, unwaxed,silk sutures are impregnated with fibroin. Single impregnation of thesutures of various sizes resulted in the following physicalcharacteristics.

Size Test All sutures are found to be non-capillary, and all exhibitexcellent bonding and hand characteristics.

EXAMPLE X Twenty parts of degummed silk cocoons are solubilizedovernight in 100 parts of a 50 percent LiSCN solution (i.e., 50 partsLiSCN and 50 parts H 0) at pH 7.0. After solubilization, the fibroinconcentrate is diluted with a 50 percent aqueous acetamide solution byusing two parts of the acetamide solution to one part fibroin-LiSCNconcentrate. The solution is filtered through cheesecloth (6 layers).

Spools with loosely-wound, non-waxed, black, braided, silk sutures, size0 and 3-0, are impregnated by the method described in Example VII. Allsamples prove to be noncapillary and exhibit excellent hand andnon-brooming characteristics.

EXAMPLE XI Size 30, black, braided, unwaxed silk is impregnated with afibroin solution utilizing LiBr instead of LiSCN as the solubilizingagent. The degummed silk cocoon is solubilized in LiBr by placing 20parts silk in parts of 10 molar LiBr which has been adjusted to pH 7.0.Following solubilization, the solution is diluted with aqueous acetamideand applied to silk sutures as described in Example X. Physical studieson the impregnated suture give the following results:

Diameter, mils. 10.0 Straight pull, lbs. 4.5 Knot pull, lbs. 3.0 Wetknot, lbs. 2.2

The suture is non-capillary and exhibits excellent physicalcharacteristics such as knot holding, lack of slippage, non-brooming,and good hand qualities.

What is claimed is:

1. A surgical silk suture comprising a plurality of silk filaments, thattogether form a strand, said silk filaments being coated with fibroinand characterize-d by non-capillarity, non-brooming, improved knotretention, and good hand qualities.-

2. The suture of claim 1 wherein the filaments are bonded together bythe fibroin.

3. The suture of claim 1 wherein the fibroin partially fills theinterstices present betwen the individual filaments that form thestrand.

4. The suture of claim 1 wherein the fibroin amounts to about 2 percentby weight of the strand.

References Cited UNITED STATES PATENTS 1,992,202 2/1935 Furmam et a1.106-161 1,989,005 1/1935 Fink et al. 106-161 2,193,188 3/1940 Bradley128-3355 2,357,503 9/1944 Cidonio 57-157 3,187,752 6/1965 Glick 128-3355DALTON L. TRULUCK, Primary Examiner.

J. YASKO, Assistant Examiner.

US. Cl. X.R.

